The present invention relates in general to the detection of fluid and other materials in biological tissues and, more particularly, to electromagnetic sensors and methods of using those sensors for the detection of fluid and other materials in tissues. While the present invention is generally applicable to sensing of bodily or injected fluid levels or the presence of other foreign materials, such as tumors, in tissues, as well as microwave imaging and the like, it will be described herein primarily with reference to extravasation for which it is particularly applicable and initially being utilized.
Sensing changed, elevated or abnormal fluid levels in living tissues is often important to patient treatment. One example of abnormal fluid levels in tissue is edema, i.e., an abnormal accumulation of watery fluid in the intercellular spaces of connective tissue. Edematous tissues are swollen and, when punctured, secrete a thin incoagulable fluid. Edema is most frequently a symptom of disease rather than a disease in itself, and it may have a number of causes, most of which can be traced back to gross variations in the physiological mechanisms that normally maintain a constant water balance in the cells, tissues, and blood. Among the causes may be diseases of the kidneys, heart, veins, or lymphatic system; malnutrition; or allergic reactions. Abnormal fluid levels also arise in tissues due to hemorrhage or the discharge of blood from blood vessels with the collection and clotting of blood in tissues leading to hematomas. Hematomas normally are the result of injury.
In addition to the accumulation of body fluids, elevated fluid levels in tissues can arise as a result of introduction of a fluid into the body, for example, during an injection procedure. In that regard, in many medical diagnostic and therapeutic procedures, a physician or other person injects fluid into a patient's blood vessels. Moreover, in recent years, a number of injector-actuated syringes and powered injectors for pressurized injection of contrast medium in procedures such as angiography, computed tomography, ultrasound and nuclear magnetic resonance/magnetic resonance imaging (NMR/MRI) have been developed.
Extravasation or infiltration is the accidental infusion or leakage of an injection fluid, such as a contrast medium or a therapeutic agent, into tissue surrounding a blood vessel rather than into the blood vessel itself. Extravasation can be caused, for example, by rupture or dissection of fragile vasculature, valve disease, inappropriate needle placement, or patient movement resulting in the infusing needle being pulled from the intended vessel or causing the needle to be pushed through the wall of the vessel. High injection pressures and/or rates of some modern procedures can increase the risk of extravasation. In computed tomography, for example, contrast injection flow rates can be in the range of 0.1 to 10 ml/s.
Extravasation can cause serious injury to patients. In that regard, certain injection fluids such as contrast media or chemotherapy drugs can be toxic to tissue. It is, therefore, very important when performing fluid injections to detect extravasation as soon as possible and discontinue the injection upon detection.
Several extravasation detection techniques are known in the art. Two simple and very useful techniques for detecting extravasation are palpation of the patient in the vicinity of the injection site and simple visual observation of the vicinity of the injection site by a trained health care provider. In the palpation technique, the health care provider manually senses swelling of tissue near the injection resulting from extravasation. By visual observation, it is also sometimes possible to observe directly any swelling of the skin in the vicinity of an injection site resulting from extravasation.
In addition to palpation and observation, there are a number of automatic methods of detecting extravasation that include automatically triggering an alarm condition upon detection. For example, U.S. Pat. No. 4,647,281 discloses subcutaneous temperature sensing of extravasation to trigger such an alarm. In this method of extravasation detection, an antenna and a microwave radiometer instantaneously measure the temperature of the subcutaneous tissue at the site where fluid is injected. An algorithm periodically determines the temperature difference between tissue and injected fluid, and compares the difference to a fixed threshold. An alarm processor uses the comparison to determine an alarm condition.
U.S. Pat. No. 5,334,141 discloses a microwave extravasation detection system employing a reusable microwave antenna and a disposable attachment element for releasably securing the microwave antenna to a patient's skin over an injection site. The attachment element holds the antenna in intimate contact with the patient's skin to optimize microwave transfer therebetween, while shielding the antenna from environmental noise signals. U.S. Pat. No. 5,954,668 also discloses use of a microwave antenna to sense temperature of tissue to detect extravasation.
In addition to microwave radiometry for the detection of extravasation as described above, radiometry has also been proposed for the detection of pulmonary edema as described in U.S. Pat. No. 4,488,559. U.S. Pat. No. 4,240,445 discloses detection of pulmonary edema via transmitting electromagnetic energy through a transmission line coupled to tissue. U.S. Pat. No. 4,690,149 discloses detection of brain edema via impedance changes detected by a sensor. A proposed method of detection of brain edema is also disclosed in U.S. Pat. No. 6,233,479, in which a measured signal from a microwave antenna is compared to stored characteristic hematoma signals from hematomas of different thicknesses and a predetermined threshold value which can be used for judging whether or not a hematoma signal from an actual patient represents a real blood pool or not.
Microwave energy has also been used for the detection of tumors in living tissue as described in U.S. Pat. No. 6,061,589. Unlike the passive measurements in microwave radiometry, U.S. Pat. No. 6,061,589 disclosed transmission of electromagnetic energy into the body (breast tissue) using a microwave antenna with collection and measurement of a resultant signal. In that regard, U.S. Pat. No. 6,061,589 describes a microwave antenna to detect incipient tumors in accordance with differences in relative dielectric characteristics. Electromagnetic energy in the microwave frequency range is applied to a discrete volume in the tissue and scattered signal returns are collected. The irradiated location is shifted or changed in a predetermined scanning pattern. The returned signals are processed to detect anomalies indicative of the present of a tumor.
Likewise, microwave energy has been proposed for use in water content mapping in human tissue as described in U.S. Pat. No. 5,995,863. Microwave energy has also been used in non-invasive tomographic spectroscopy imaging. See U.S. Pat. Nos. 6,332,087 and 6,026,173.
Microwave energy has also further been used to measure the fat content in nonliving organic tissue. For example, M. Kent, “Hand Held Fat/Water Determination”, (1993), available at www.distell.com/products/papers/paper2.htm, discloses a microstrip transmission line type sensor for such a determination. In general, the fat content of pelagic and other fatty species of fish is proportion to water content. The dielectric properties of the fish depend on the water content. In the device of Kent, changes in the transmission properties of the microstrip transmission line held against the fish were calibrated against water content. Through simulations it was found that the present invention is significantly more sensitive to changes within biological tissue due to the fact that it relies on the tissue as the transmission path rather than a transmission line. In an open transmission line type sensor, a significant fraction of the transmitted energy travels through the transmission line itself and is significantly less impacted by changes in the underlying tissue. In the present invention, a large fraction of the energy travels through the tissue and therefore changes in the tissue path will impact the signal more drastically.
It is very desirable to develop improved sensors and methods for their use in detecting elevated or otherwise abnormal levels of fluids in living tissue, for example, as the result of edema, hematoma or extravasation. Such sensors and methods would also be desirable for detecting the presence of other materials, such as tumors, in living tissue as well as for microwave imaging and other like applications.